1. Field of the Invention
The present invention relates to thermal air flow meters suited to measure intake air flow rates of internal combustion engines, for example.
2. Description of the Related Art
A thermal air flow meter capable of directly measuring a mass flow rate has been widely used in an electrically-controlled fuel injection system for an engine of a motor vehicle etc; however, a pulsation flow accompanied with a reverse flow takes place under the condition of a heavy load as well as low engine rotation speed, thereby causing a large error in a conventional thermal air flow meter incapable of detecting the reverse flow. In order to reduce the error in measuring this pulsation flow accompanied with the reverse flow, a method has been proposed in which an air flow direction is detected and an intake air flow rate signal is corrected only when the reverse flow is detected.
For example, Patent Document 1 describes a method by which a signal related to the air flow direction is generated from difference in potential between an upstream resistor and downstream resistor of a heating resistor, and while this direction signal is indicating a reverse flow, the flow rate signal is reversed and counted as a negative value. This signal processing is outlined in FIG. 28, in which the flow rate signal is reversed in a region where the direction signal is ‘Low’ (region of reverse air flow taking place), so as to become negative.
Moreover, Patent Document 2 also describes a method of detecting the air flow direction. As shown in FIG. 29, a constant-temperature-difference drive bridge circuit is configured using a heating resistor 104, a temperature-compensating resister 107 and a plurality of fixed resisters, and the potential at a midpoint of the bridge circuit is taken out as a flow rate signal. On the other hand, a bridge circuit is configured using an upstream temperature detection resistor 105 located upstream of the heating resistor 104 and a downstream temperature detection resistor 106 located downstream of the heating resistor 104, and potential difference between the midpoints of the bridge circuit is taken out as a direction signal. The flow rate signal and direction signal are input to a microcomputer 115; when it is determined that a reverse flow is taking place, twice the flow rate signal during the period of the reverse flow taking place is subtracted from the flow rate signal so as to correct the signal.
Moreover in Patent Document 3, when it is determined based on the direction signal that a reverse flow is taking place, twice the flow rate signal during the period of the reverse flow taking place is similarly subtracted from the flow rate signal so as to correct the signal (see FIG. 30).
Patent document 1: Japanese Patent Publication No. 3470620
Patent document 2: Japanese Laid-open Patent Publication No. H08-14978
Patent document 3: Japanese Laid-open Patent Publication No. H08-43163
A method of correcting the flow rate signal based on the direction signal is adopted in the foregoing conventional example, so the method of processing the flow rate signal needs to be selectively switched over depending on which is taking place, a forward flow or reverse flow, which consequently requires a switch that is switched over in accordance with the direction signal and a microcomputer and the like that determine the direction. The signal processing is carried out in an engine control circuit in Patent document 3, and it is assumed also in Patent documents 1 and 2 to use a microcomputer for the processing. Therefore, a problem has been that when the same processing as that in the conventional example is carried out, the configuration becomes complicated and the circuit size increases. In addition, there has been another problem in that its cost is thereby also pushed up.
On the other the hand, for the purpose of improving fluctuation of characteristics (contamination characteristics) when fine dust sticks to a detection element, protecting the detection element from damaging due to coarse dust colliding with the detection element or backfire from the engine, making correction for lean output under high-frequency pulsation, etc., there has been widely used a thermal air flow meter configured in such away that a sub-passage called a bypass is provided in a main passage and the detection element is disposed inside this bypass. This bypass is a passage whose inlet is connected with its outlet with a curved passage, and in most cases the relation between a forward flow rate and output (forward flow characteristics) differs from that between a reverse flow rate and output (reverse flow characteristics).
Therefore, when the bypass is adopted, a dedicated map for the reverse flow needs to be prepared in order to obtain a flow rate during the reverse flow taking place, thereby causing a problem in that its system becomes complicated. Moreover, another problem has been that when the processing is carried out using only a forward flow map without preparing the reverse flow one, accurate correction cannot be made only by subtracting from the flow rate signal twice the flow rate signal during the reverse flow taking place.